Shown in FIG. 1 is the schematic construction of a conventional apparatus in which the NMR signals due to specified atomic nuclei such as the atomic nuclei of hydrogen in an object can be measured while generating both a uniform static magnetic field and a gradient magnetic field.
Referring to FIG. 1, an oscillator 1 generates high frequency pulses which are applied to a coil system 2 positioned around the object 3 in order to detect the NMR signals to be issued therefrom. Coil system 2 may be formed with the coils of a shape as shown in FIG. 5 of U.S. Pat. No. 4,254,778 issued to Clow et al. An amplifier 4 amplifies those NMR signals and analog-to-digital converter 5 converts the amplified NMR signals to digital ones. A reconstructor 6 reconstructs the tomographic image of the object on basis of those digital NMR signals, the resultant tomographic image being depicted on a display 7. Cylindrical magnetic field coils 8a, 8b, 8c and 8d are supplied D.C. voltage by a stabilized source 9 to generate a uniform static magnetic field for applying it to the object 3. Gradient magnetic field coils 10 being similar to ones shown in FIG. 3 of the above-mentioned U.S. patent, generate a gradient magnetic field in a plane perpendicular to the static magnetic field direction, whose gradient direction is shifted rotatively. The gradient field coils 10 are supplied D.C. voltage by a source supply 11, the timing of all the NMR diagnostic apparatus being controlled by a controller 12.
The operation of the above constructed NMR diagnostic apparatus will be explained as follows.
The uniform static magnetic field (Ho) due to the cylindrical magnetic field coils 8a to 8d is applied to the object 3 and the gradient magnetic field due to gradient magnetic coil 10 is also applied in a particular direction to the object 3 superimposed on the static field. While rotating the gradient direction of the gradient field in the slice of object 3 through 360.degree., high frequency pulses whose central frequency is adjusted to be the resonance frequency are issued from oscillator 1 to apply to the object 3 through coil system 2. The resultant NMR signals are detected by coil 2 and after amplification due to amplifier 4, converted to digital signals by an analog-to-digital converter 5. Since these digital signals are in correspondence to the Fourier-transformed original projection signals, after Fourier-transforming for every gradient direction of the gradient magnetic field in reconstructor 6, a reconstruction procedure is performed. The resulting tomographic image of object 3 obtained by such a procedure is depicted on display 7.
However, the above-mentioned NMR diagnostic apparatus has the following disadvantages. For both the NMR diagnostic apparatus and also tomographic apparatus utilizing such a reconstruction method, after the acquisition of signals in a variety of directions in the slice of the object, the reconstruction process is performed to obtain tomographic images. In the case of X-ray computed tomographic (CT) apparatus, since the detected signals are obtained within a few seconds after performing the X-ray beam scan, the time from starting the scan to completing the display of a tomographic image is mostly occupied with the calculation time involved in the reconstruction procedure, its calculation time being as short as 30 seconds. Accordingly, even if the desired tomographic image was not obtained because of an error in slice positioning, body-motion of the object during the scan or the like, remeasurement for an X-ray CT apparatus can be performed without undue delay.
However, in the case of NMR diagnostic apparatus the detection of NMR signals requires several minutes. Accordingly, the total time of diagnosis could be excessive if remeasurement after performing the reconstruction procedure is required on the basis of the detected NMR signals and display of the tomographic image.
Furthermore, there is a diagnostic method of obtaining a tomographic image of the object after injecting a contrast medium into the object and reconstructing the same slice many times. The alteration of various organs in the same slice with the lapse of time can thereby be observed. If such a diagnostic method is performed using NMR diagnostic apparatus, the safety of the object is greater than if an X-ray CT apparatus is used because ionizing radiation exposure is nonexistent. However, using prior art systems several minutes for detection of NMR signals are necessary for the reconstruction of a tomographic image as previously described, and it is consequently impossible to perform this type of diagnostic method with the NMR diagnostic apparatus.
It is therefore an object of the present invention to provide a nuclear magnetic resonance diagnostic apparatus wherein a rough tomographic image is reconstructed before acquiring a final tomographic image, thereby enabling a quick decision to be made whether a remeasurement is necessary and also to observe a time-lapse change in an object's slice without excessive and disruptive delays.
Additional objects and advantages of the invention will be set forth in the description which follows, and will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized by means of the apparatus and combinations particularly pointed out in the appended claims.